Audio spatialization is of interest to many headphone users, such as gamers (where is my opponent?), audiophiles (where is the cello?), and pilots (where is ground control?), for example. Location cues can be rendered through conventional headphones to signal, for example, the location of an opponent's footsteps in a video game. The normal human array of two ears, the complex shape of the pinnae, and the computational capacities of the rest of auditory system provide sophisticated tools for sound localization.
These tools include head related transfer function (HRTF), which describes how a given sound wave input (parameterized as frequency and source location) is filtered by the diffraction and reflection properties of the head, pinna, and torso, before the sound reaches the transduction machinery of the eardrum and inner ear; interaural time difference (ITD) (when one ear is closer to the source of the sound waves than the other, the sound will arrive at the closer ear sooner than it will at the ear that is farther from the sound source); and interaural level difference (ILD) (because sound pressure falls with distance, the closer ear will receive a stronger signal than the more distant ear). Together these cues permit humans and other animals to quickly localize sounds in the real world that can indicate danger and other significant situations. However, in the artificial environment of reproduced sound, and particularly sound reproduced through headphones, localization can be more challenging.
Presenting additional information through taction can provide another means for enhancing the perception of sound location.